Two of the most widely used processes for removal of H.sub.2 S from process gas streams are the catalytic processes that use 1) an iron chelate catalytic absorption solution, e.g., the LO-CAT.RTM. process and 2) a metal vanadate catalytic absorption solution, e.g., the Stretford process. The oxidation-reduction reactions that permit these processes to be carried out continuously are well known to those skilled in the H.sub.2 S removal art and are well documented in the literature. The ferric iron chelate-H.sub.2 S reactions can be represented as follows: EQU H.sub.2 S(gas)+H.sub.2 O(Liquid).revreaction.H.sub.2 S(aqueous)+H.sub.2 O(Liquid) EQU H.sub.2 S(aqueous).revreaction.H.sup.+ +HS.sup.- EQU HS.sup.- .revreaction.H.sup.+ +S.sup.= EQU HS.sup.- +2(Fe.Chelate).sup.+3 .fwdarw.H.sup.+ +S.degree.+2(Fe.Chelate).sup.+2 or EQU S.sup.= +2(Fe.Chelate).sup.+3 .fwdarw.S.degree. (solid)+2(Fe.Chelate).sup.+2
By combining these equations, the resulting equation is: EQU H.sub.2 S(gas)+2(Fe.Chelate).sup.+3 .fwdarw.2H.sup.+ +2(Fe.Chelate).sup.+2 +S.degree.
In order to have an economical, workable process to effect catalytic oxidation of the hydrogen sulfide using an iron chelate redox solution, it is essential that the hydrogen sulfide gas be brought continuously into intimate contact with the chelated iron solution and that the ferrous iron chelate formed in the above described manner be continuously regenerated by oxidizing to ferric iron chelate by intimate contact with dissolved oxygen, preferably in the form of ambient air. The series of reactions that take place when regenerating the required ferric iron chelate can be represented by the following equations: EQU O.sub.2 (gas)+2H.sub.2 O).fwdarw.O.sub.2 (aqueous)+2H.sub.2 O EQU O.sub.2 (aqueous)+2H.sub.2 0+4(Fe.Chelate).sup.+2 .fwdarw.4(OH.sup.-)+4(Fe.Chelate).sup.+3
By combining these equations the resulting equation is: EQU 1/2O.sub.2 (gas)+H.sub.2 O+2(Fe.Chelate).sup.+2 .fwdarw.2(OH).sup.- +2(Fe.Chelate).sup.+3
The economics and workability of the Stretford process have depended upon a large volume of the metal vanadate solution. The reduced metal vanadate, after absorption of the H.sub.2 S (as HS.sup.- or S.sup.=) to form the metal vanadate in the +4 valance state is continuously regenerated to the +5 valance state by contact with dissolved oxygen for continuous use of the oxidized metal vanadate in the absorption zone to remove additional H.sub.2 S as elemental sulfur. The Stretford process chemistry is typically summarized according to the following steps:
Absorption and dissociation of H.sub.2 S in alkali: EQU 2H.sub.2 S(g)+2Na.sub.2 CO.sub.3 .fwdarw.2NaHS+2NaHCO.sub.3 ;
Bisulfide oxidation with metavanadate to form elemental sulfur and reduced vanadium: EQU 2NaHS+4NaVO.sub.3 +H.sub.2 O.fwdarw.Na.sub.2 V.sub.4 O.sub.9 +4NaOH+2S; and
Vanadium reoxidation by dissolved molecular oxygen in the presence of ADA: ##STR1##
The reactions described have been carried out by means of two broad process flow schemes which may be described as "aerobic" and "anaerobic". In the aerobic flow scheme, both the absorption of H.sub.2 S and the reactions related thereto and the absorption of oxygen and the regeneration reactions related thereto take place in the same vessel through which the reactive solution is circulated continuously. In the "anaerobic" process scheme, a H.sub.2 S-containing gas which is substantially free of oxygen is treated in an anaerobic absorber vessel and the solution is subsequently contacted with air or other oxygen-containing gas stream in a separate oxidizer vessel. The aerobic configuration has the advantage of simplicity, in that only a single gas-liquid contacting vessel is required, and may be the only way a gas containing both H.sub.2 S and significant quantities of oxygen can be processed. However, the aerobic flow scheme has some substantial disadvantages, in that the co-absorption of oxygen and H.sub.2 S usually leads to the formation of relatively large amounts of undesirable water-soluble compounds of sulfur and oxygen rather than elemental sulfur.
U.S. Pat. No. 4,238,462 describes a process and method of operation whereby an anaerobic gas stream containing H.sub.2 S could be treated and the solution regenerated in a single vessel without co-absorption of H.sub.2 S and O.sub.2, and without the need for circulating pumps to circulate the redox solution between the absorption section and the regeneration section of the single vessel, described as an Autocirculation reactor.
The method of U.S. Pat. No. 4,238,462 has been commercially successful and has demonstrated the workability of the Autocirculation reactor. However, this method and apparatus suffer from several disadvantages which limit the scope of application of the Autocirculation reactor and also limit the efficiency of conversion of the H.sub.2 S to elemental sulfur rather than water soluble by-products. These disadvantages include having a very limited ability to process large volumes of H.sub.2 S contaminated gas because of the necessity of utilizing a liquid-filled absorber section with a low gas capacity per square foot of absorber area. Also, the rate of circulation of solution is set by the initial geometry of the Autocirculation reactor and can neither be measured nor controlled during operation. Finally, the Autocirculation reactor designs have generally shown relatively high thiosulfate production, in the range of 4 to 6% of the theoretical sulfur production, and relatively high losses of chelated iron with the settled sulfur product.
A great many prior art patents are directed to the removal of H.sub.2 S usng catalytic polyvalent metal redox solutions, such as an iron chelate or a metal vanadate Without exception, the prior art patents address the absorption of H.sub.2 S in solutions that contain at least the theoretical quantity of iron to provide two moles of iron for each mole of H.sub.2 S so that the conversion of absorbed H.sub.2 S to sulfur can be carried out in the absorber vessel or in a subsequent reaction chamber. Further, the presumption is implicit in prior art patents that it is necessary to provide sufficient iron to oxidize or complex with all the H.sub.2 S to prevent precipitation of the active catalyst metals as the metal sulfide. Further, it is well known to those skilled in the art of hydrogen sulfide oxidation by metal redox solution techniques that if insufficient oxidized metal is supplied to the absorber, free HS.sup.- and S.sup.= ions will exist in the solution exiting the absorber and the reaction chamber if one is provided. This will result in the non-selective reaction between HS.sup.- , S.sup.= ions and dissolved oxygen in the oxidizer vessel, and cause the production of relatively large quantities of thiosulfate ions (S.sub.2 O.sub.3.sup.=) which are quite water soluble and accumulate in the metal redox catalyst solution. Ultimately, it is necessary to withdraw some solution to limit the concentration of such by-product salts in order to avoid problems of reduced gas solubility and efficiency of absorption of both H.sub.2 S and oxygen.
In the present invention it was discovered first that catalytic polyvalent metal redox solutions can be used to absorb H.sub.2 S from gas streams at high efficiency with less than the theoretical amount of polyvalent metal for conversion of the HS.sup.- and S.sup.= to sulfur provided that the solution possesses sufficient alkalinity for adequate H.sub.2 S absorption by means of the reaction: Na.sub.2 CO.sub.3 +H.sub.2 S.fwdarw.NaHCO.sub.3 +NaHS or other non-catalytic reaction, and provided the ferrous iron chelate produced by reduction of ferric iron chelate in the presence of free HS.sup.- ions or NaHS in solution can be removed from the absorber and reoxidized quickly after processing through the reactor. To achieve the full advantage of this discovery, it was found that it is highly desirable that the residence time of the solution in the highly reduced condition (exiting the absorber) should be kept quite short (e.g., less than about 2 minutes, preferably less than about 1 minute) so as to prevent precipitation of the metal sulfide. The introduction of the solution with an excess of HS.sup.- or S.sup.= ions (over that which could be reacted with chelated iron in the solution) into the oxidizer resulted in excessively high rates of water soluble, oxygen-containing sulfur compounds such as thiosulfate and sulfate, which would, in commercial practice, require high rates of blowdown to purge these soluble compounds, and which would result in the loss of excessive quantities of chelated iron.
Secondly, it was discovered that the high thiosulfate and sulfate formation could be prevented by conducting the highly reduced spent absorber solution into a reaction chamber in which it was mixed with highly oxidized redox solution rather than conducting the spent solution from the absorber directly into the oxidizer vessel or into the reaction chamber without first admixing in with oxidized solution. Advantageously, contact of highly reduced polyvalent metal redox solution with highly oxidized polyvalent metal redox solution results in completing the oxidation of HS.sup.- and S.sup.= to elemental sulfur, in the absence of contact with dispersed air or oxygen, and without the necessity of pumping the large volume of oxidized solution up to the high pressure of the absorber vessel.
Examples of the prior art patents directed to the use of polyvalent metal redox solutions for H.sub.2 S removal include the following: Hartley, et al. U.S. Pat. No. 3,068,065; Siebeud, et al. U.S. Pat. No. 3,897,219; Salemme U.S. Pat. No. 3,933,993; Meuly U.S. Pat. No. 4,009,251; Mancini, et al. U.S. Pat. No. 4,011,304; Thompson U.S. Pat. No. 4,189,462; Hardison U.S. Pat. No. 4,238,462; Blytas, et al. U.S. Pat. No. 4,356,155; Hardison U.S. Pat. No. 4,482,524; McManus, et al. U.S. Pat. No. 4,622,212; Primack, et al. U.S. Pat. No. 4,455,287; Fong, et al. U.S. Pat. Nos. 4,664,902 and 4,705,676.
One of the most significant problems in the removal of H.sub.2 S gas using a catalyzed polyvalent metal redox solution, particularly either an iron chelate redox absorption solution or a vanadium-based redox absorbtion solution, is that the efficiency of the redox reactions required of polyvalent metal-redox solutions is somewhat pH dependent. It is well known that polyvalent metal redox solutions are capable of solubilizing the contaminant metal ions at a pH well above pH 7, but the rate of absorption of H.sub.2 S decreases substantially with decreasing pH, despite statements in issued patents to the effect that a broad range of pH is acceptable--e.g., see Hartley U.S. Pat. No. 3,068,065; Pitts, Jr., et al. U.S. Pat. No. 3,097,925; Meuly, et al. U.S. Pat. No. 3,226,320; Roberts, et al. U.S. Pat. No. 3,622,273. Others have recognized that periodic addition of alkali is needed to maintain a suitably high pH for good absorption efficiency--e.g., see Roberts et al. U.S. Pat. No. 3,622,273, since the pH tends to drop as the reactions proceed.
As described in the Meuly U.S. Pat. No. 4,009,251, it is recognized that the pH of polyvalent metal redox solutions is lowered during the H.sub.2 S removal (absorption) redox reactions because of other side reactions between the redox solution and the H.sub.2 S and the resulting formation of acidic salts. As recognized in the Meuly U.S. Pat. No. 4,009,251, these acidic side reaction products are for the most part oxides of sulfur represented by the formula S.sub.x O.sub.y, where x is generally 1 or 2; and y is generally 2 or 3, that are present in an alkali-containing redox solution as predominantly sulfates and thiosulfates.
The more acidic salts that are formed in the polyvalent metal catalytic redox solution as a result of a relatively high pH, e.g., above 7, and particularly between about 8 and 9.5, the more frequent it is necessary to add alkali periodically to maintain the desired relatively high pH. As a result, more acidic salts are formed in the redox solution thereby requiring a periodic "blowdown" of polyvalent metal chelate solution (a term used to denote the irretrievable discarding of some or all of the polyvalent metal redox solution and replacement with fresh, non salt-contaminated solution). Since the polyvalent metal redox solutions are relatively expensive, the efficiency of the redox reations catalyzed by polyvalent metal redox solutions at a relatively high pH must be balanced by the expense of the addition of alkali and the expense of lost solution because of "blowdown" being necessary periodically to maintain acidic salt concentration in the catalytic redox solution below an acceptable upper limit. Further, the acidic sulfur salts formed during H.sub.2 S absorption necessarily reduce the elemental sulfur yield from the H.sub.2 S removal process.
One method disclosed useful to substantially inhibit salt formation in a polyvalent metal redox solution in a process for the catalytic removal of H.sub.2 S from a process gas is disclosed in the Meuly U.S. Pat. No. 4,009,251, using particular polyvalent metal chelating agents to inhibit oxidation of sulfur beyond elemental sulfur. In accordance with the present invention, it has been found that thiosulfate concentration in the polyvalent metal redox catalytic solution can be controlled with a minimum of loss of polyvalent metal catalyst while reducing the size and cost of the equipment necessary for removing H.sub.2 S from both low pressure and high pressure gas streams.
In accordance with the process and apparatus of the present invention, experiments have shown that new and unexpected results, such as greatly reduced pump horsepower and mechanical losses of catalyst, as well as low thiosulfate production in the polyvalent metal redox solution are achieved by operating the absorber with much less than the theoretical amount of iron, and discharging the spent solution, having an excess of dissolved sulfide (HS.sup.-) and bisulfide (S.sup.=) ions into a reaction chamber disposed in the process scheme between an absorber chamber and an oxidizer of the process. In this manner, H.sub.2 S-laden polyvalent metal redox solution, containing dissolved HS.sup.- and S.sup.=, is contacted with polyvalent metal redox solution from the last oxidizer stage, containing relatively highly oxidized polyvalent metal redox solution and substantially zero dissolved oxygen, for very rapid formation of elemental sulfur in the reaction chamber. A portion of partially oxidized polyvalent metal redox solution from the reaction chamber then flows to the absorber vessel for further absorption of H.sub.2 S from the process gas and the remainder of the partially oxidized polyvalent metal redox solution proceeds through the oxidation stage(s) for recirculation to the reaction chamber after complete oxidation.
Alternatively, some or all of the solution leaving the reaction chamber can flow to the oxidizer vessel for regeneration, and a portion of this relatively highly oxidized solution can be pumped into the absorber, while a majority of the solution is recycled to the reaction chamber and by-passes the absorber.
In either of these methods, there is a substantial saving in pump horsepower over a conventional redox process in which all of the regenerated solution is pumped through the absorber.